Ah right, it's good that you asked that because I didn't pick up on that, due to the fact I don't know the area that well.
With regards to doubling back there is a concession as shown below.
GROUP STATIONS
Some stations are grouped together to improve interchange between trains
by offering customers access to a wider choice of train services and station
facilities. A customer may travel via any station in such a group, including
doubling back, provided that the group is on one of the permitted routes
between their origin and destination stations. This extended availability is for
interchange purposes only and does not apply where the origin or destination
stations are part of a group.
However on this occasion as Langley is in Slough Group, that double back concession wouldn't seem to apply as the place you double back has to be outside the group which the origin station/destination station are part of, therefore doubling back between Slough and Langley would only be permitted if there is an easement in the routing guide.
Having looked in the Easements section of the routing guide I can see no such easements, therefore I would say, that the only permitted routing point for Langley is Hayes & Harlington.